I'm not planning on changing the hardware, partly because I'm happy enough with it and because of a tight budget.

The board has a bug that ASUS are being quite obtuse about, so I guess I'll have to accept the status quo. When this board resumes from standby/S3/sleep, the undervolting setting I've set in the BIOS is forgotten about until the PC is either restarted, cold-started or resumed from hibernation.

I use the computer several times a day, and perhaps for hours at a time, or if I'm out on appointments I might use it several times a day for only say an hour on 2-3 occasions.

The undervolting setting isn't much, but it is the maximum under-volt setting that the board's options will let me choose. When I had all six cores enabled, here's the power usage stats:

My feeling is that 5W saved on idle isn't much, but over a year if it saved a fiver it's better than no saving at all. However, the power usage penalty of resuming from hibernation three times a day might offset that. This is really the crux of my question. When a computer starts up (I imagine hibernation and a cold start are fairly similar in terms of their demands power-wise), the processor is cranked up quite a bit until the system finishes booting.

Currently I've got the computer going into hibernation after an hour of idle time (I'm not sure why I changed it from 45 minutes to an hour now, as 45 minutes, but anyway).

The advantage of going back to using S3 sleep is obviously that it resumes a heck of a lot quicker - (5 seconds versus 30 for hibernation), and probably uses less power to resume from sleep than hibernation.

My question was more along the lines of should I bother with undervolting when I have to use hibernate rather than use S3 sleep.

Your computer uses an extra 5 W-hours for every hour it's on and not undervolted. It probably uses about 50 W-seconds to pull itself out of hibernation. Absolutely use hibernation rather than sleep to save energy, unless the bootup time annoys you too much.

Let's say that you use the computer for three 2 hour sessions during the day. Let's assume that most usage is fairly light so that your average power usage is basically the idle measurement. That gives us 90W idle when undervolting and 95W idle when running at stock volts. Let's assume that you use full power (186W) for 30 seconds when powering the machine up. Let's also assume that the machine uses 5W while sleeping and 1W while hibernated.

If you use your machine for an average of say 6 hours a day, then you will hibernation and sleep are equal if that usage is broken up into 188 difference sessions. If you have fewer sessions, then hibernation wins, more sessions and hibernation loses.

So it's pretty obvious that for any sane usage pattern, hibernation is much more energy efficient.

PS. I did this really quick, so anybody feel free to point out algebraic or arithmetic errors.